Agents targeting topoisomerase I or II are active against a wide range of human tumors. Stabilization of covalent complexes, converting topoisomerases into DNA damage, is an essential aspect of cell killing by these drugs. Genetic experiments have demonstrated that enzymes that participate in the excision repair pathway are capable of interacting with the covalent complexes formed by either topoisomerase I or II. Unexpectedly, mutations in excision repair genes lead to partial resistance to topoisomerase agents, suggesting that processing of covalent complexes is related to the cytotoxicity of topoisomerase targeting drugs. The experiments in this grant will examine the consequences of mutations in various DNA repair pathways to determine how DNA repair pathways process topoisomerase-mediated DNA damage. Biochemical experiments will test which components of the excision repair machinery recognize and process topoisomerase-mediated DNA damage. Genetic approaches will be used to identify other DNA repair enzymes that influence cell killing by anti-topoisomerase agents. The processing of topoisomerase:DNA covalent complexes by DNA repair proteins may also play a role in the other cellular effects of topoisomerase targeting drugs. Recent evidence has demonstrated that etoposide and other topoisomerase II-targeting drugs can cause secondary malignancies by generating translocations that cause the overexpression and activation of proto-oncogenes. Since camptothecins also generate high levels of recombination, topoisomerase I targeting drugs may also lead to significant levels of secondary malignancies. A major aim of this project is will be to apply a yeast model system to study how DNA repair functions influence the type and frequency of genetic aberrations caused by drugs targeting topoisomerases I or H. These experiments will provide information concerning how topoisomerase inhibitors exert their cytotoxic effects against cancer cells, and how cancer cells may develop resistance to these agents. The experiments will also be important for understanding how topoisomerase targeting drugs lead to secondary malignancies.